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Seeds and Seed Germination
Seeds
Objectives of today’s lecture:• Learn about the structure and
composition of seeds• Learn how seeds are used in horticulture• Learn what happens during germination
of seeds and the factors that influence this process
Seeds
• Seeds are normally the product of sexual reproduction
Pollination
Fertilization
Embryogenesis
Mature seed
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Seeds
• Some seeds are produced without pollination, called apomixis. Examples include many citrus crops, mango, Kentucky bluegrass
• The plants produced from apomictic seed are genetically identical to the maternal plant - clones
Pollination
Fertilization
Embryogenesis
Mature seed
Seeds are for propagation
• The biological function of seeds is for propagation of the species• This is also one of the
major functions of seeds in horticultural practice
• What else are seeds used for?
Seeds are the major source of food for the world
• Cereal crops: wheat, rice, corn, barley, oat, rye, millet, sorghum
• Pseudo cereals: buckwheat, amaranth• Oil crops: soybean, sunflower, canola,
other mustards• Pulses: lentils, chickpeas• Beans: navy, pinto kidney, fava, lima,
butterbean, pigeon pea
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Top US seed Crops (2001)
• Corn- 9.51 billion bushels• Sorghum (for grain) – 515 million bushels• Rice – 213 million hundred weight• Soybeans- 2.89 billion bushels• Wheat 1.4 billion bushels• Oilseed- 80 million tons
Seeds are alive!
• To function in propagation, seeds must be alive
• Seeds respire (slowly)• consume O2, produce CO2 and H2O
• Seeds have a finite lifespan• They cannot be stored indefinitely
Seeds are alive!
• Seeds of many tropical plants remain viable for only a short time, a few days• Tropical plants grow in environments that do not have
a winter season through which seeds must survive before the spring growing season. There is a cost in dormancy.
• Other seeds remain viable for a very long time, in some cases more than 100 years• Common feature of many weeds• Seed storage experiment at Michigan State
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General features of seeds
A number of structural features are common to almost all seeds
• Embryonic axis• Root and shoot, in a miniature form
• Food reserves• Allow seedling to grow before it is capable
of performing photosynthesis• Seed coat
• Provides protection from the environment
Monocots and Dicots
• Flowering plants (angiosperms) are divided into 2 groups based on seed structure• Dicotyledonous plants with two seed leaves• Monocotyledonous plants with one seed leaf
Angiospermsflowering plants
Dicots200,000 species
Monocots50,000 species
includes beans, roses, cacti, melons, citrus
includes grasses, lilies, orchids, palms
Monocots and Dicots
• In addition to differences in seed morphology, there are a number of other common differences between monocots and dicots
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A typical seed of a dicotyledon
• Embryonic axis (plant in miniature)• Plumule - first true
leaves• Hypocotyl/Epicotyl -
embryonic stem (H/E)• Radicle - embryonic
root
A typical seed of a dicotyledon
• Cotyledons (seed leaves for storage of food reserves)• Proteins• Starches,
carbohydrates• Lipids, oils
A typical seed of a dicotyledon
Exterior structure• Seed coat for protection• Hilum
• where seed was attached to mother plant, “botanical belly button”
• Micropyle• where tube that carried
pollen to the egg was attached
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A typical seed of a monocotyledon
Embryonic axis• Plumule - first true
leaves• Radicle - embryonic
rootColeoptile• Protective cap over
plumule
A typical seed of a monocotyledon
Scutellum• Transfer of food from
endosperm to seedling
Coleoptile and scutellumare equivalent to cotyledons in a dicot
A typical seed of a monocotyledon
Endosperm• Food reserve and
storage• Proteins, oils and
starches
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Diversity among seeds
• Seeds are very diverseIn terms of size• Begonia and Impatiens
seed weigh 10-20 micrograms (millionths of a gram)
• Coconuts weigh more than a kilogram, seeds of related palms weigh more than 15 kgs
Diversity among seeds
In terms of adaptation• To survive various environments until
conditions are favorable for germinationIn terms of method of distribution• By animals, wind, water
Diversity among seeds
Distribution of coconut seeds by water
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Seed dormancy
• Maintains seed during adverse growth conditions (cold, drought)
• Lower metabolic rate• Energy cost to reverse dormancy• Maintained by hormone abscisic acid
(ABA) / reversed by hormone gibberelicacid (GA)
• Stratification• Scarification
How long can a seed remain dormant
500 year old lotus seeds germinated!
Other seeds last a year or less
Germination of seeds
A complex series of steps involving:A. Uptake of waterB. Utilization of stored reservesC. Development and expansion of the
embryonic axisD. Establishment of a seedling capable
of sustained, independent growth
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Germination of seeds
A.A. Uptake of water - imbibition
• Seeds are normally desiccated (~10% water)
• Desiccation allows seed to remain dormant
• Water uptake starts process of germination
Germination of Seeds
Other important factors to consider at beginning:
• Temperature requirement• Some seed require a minimum temperature to
germinate, e.g. tomato will not germinate below 10˚C (50˚F)
• Increased respiration• More oxygen is required for metabolism
B.B. Utilization of stored reserves• In cotyledons or endosperm tissue• During germination, enzymes are made that
convert stored reserves (large molecules) into compounds that can be used by the seedling (smaller molecules)• starches → sugars• lipids, fats → sugars• proteins → amino acids
Germination of seeds
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Germination of seeds
C.C. Expansion and growth of seedling• Root radicle elongates down, hypocotyl
expands up• Establishment of root system and emergence
of shoot • Transport of compounds into growing
seedling through vascular system• These compounds have two functions
• Support respiration in the embryo• Provide a source of building blocks (carbon,
nitrogen, etc.) for the seedling
Germination of Seeds
D.D. Establishment of Seedling- will discuss next lecture in more detail
Activity
• Living Necklace (aka –baby plant)
• You will germinate a seed• Hint – you get to care for and observe it in the process!
• You will need:• 1 Bag• 1 String• 1 Cotton ball• Few drops water• 1-2 Seeds
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Germination of dicot seeds
Germination of monocot seeds
Seed germination and the conversion of starches into sugars is important
Mature barley plants
Barley grain
The malting processBarley is germinated and enzymes produced in the seed convert starches to sugars
Germination is arrested by heating and
drying the grain. The dry malt is
the starting material for
brewing
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The importance of converting starches into sugars during germination
Sugars are extracted from the malt and mixed with yeast, hops and other flavoring ingredients. During fermentation, the sugars produced during germination of the barley grain are converted to alcohol
Conclusions
• Seeds are alive but dormant• Comprise an embryonic plant and stored
reserves• Germination requires
• Water - for imbibition• Oxygen - for respiration• Suitable temperature
• Outcome of successful germination is a seedling capable of independent growth